Cardiac hypertrophy involves the expression of a gene program that occurs when cardiomyocytes are continuously exposed to stresses such as mechanical stretch or neurohumoral stimulation. The enlargement initially improves cardiac function; over time, however, this compensatory hypertrophy predisposes individuals to arrhythmias, pathological hypertrophy and ultimately heart failure. Numerous genetic and pharmacological studies have demonstrated that oxidative stress is intimately involved in the progression of cardiac hypertrophy and heart failure. However, the molecular mechanisms whereby cellular reactive oxygen species (ROS) contribute to the progression of pathological hypertrophy and heart failure are largely unknown. Our studies have focused on the role of Protein Tyrosine Phosphatases (PTPs) and their distinctive ability to be specifically reversibly regulated by redox signaling. This research proposal builds upon my novel observations that oxidation and inactivation of PTP1B leads to phosphorylation and inactivation of its substrate Argonaute 2 (Ago2) ? a key mediator of the biological functions of microRNAs (miRNAs) ? in the hypertrophying myocardium. We previously reported that this signal transduction pathway involving Ago2 tyrosine-393 phosphorylation prevented miRNA loading on Ago2 and led to major changes in post-transcriptional repression and oncogene-induced senescence. We have considerable preliminary data supporting that PTP1B is reversibly inactivated in cardiomyocytes and in the myocardium undergoing hypertrophy; that PTP1B inactivation is deleterious to cardiac function; and identified a signalling pathway, downstream of Ago2- phosphotyrosine 393 and PTP1B oxidation, controlling the maladaptive response to PTP1B inactivation. In this application, we test the hypothesis that by inhibiting post-transcriptional regulation of mRNAs, Ago2 tyrosine- 393 phosphorylation contributes to the regulation of cardiomyocyte hypertrophy. We propose that perturbation of the delicate balance between the action of PTP1B and an unidentified protein tyrosine kinase is implicated in the inactivation of a subset of Ago2, directly leading to changes in miRNA-mediated gene silencing involved in pathological hypertrophy. We will test our hypothesis by addressing these 2 Aims: 1) To Investigate the Requirement of PTP1B Inactivation in Cardiac Remodeling In Vivo. 2) To Investigate the Mechanisms Underlying PTP1B-Ago2 Signaling in Cardiomyocytes. We possess the expertise to explore whether the Ras-PTP1B-Ago2 pathway is involved in cardiac remodeling and all of the required reagents, models, and methodologies are in place. Importantly, it is anticipated that these studies will provide novel mechanistic information on the ROS-PTP1B-Ago2 cascade in the heart, identify novel targets for therapeutic intervention in heart failure and suggest new therapeutic strategies to prevent Ago2 inactivation in vivo.